pts.cabal: Change main module after Cabal REPL

This changes the prompt after Cabal REPL
from
    *Control.Monad.Assertions>
to
    *PTS.Dynamics>

Fix #62.

I also tried adding :load commands to GHCi, but they're executed before
the loads caused by cabal REPL and slow down the loading. :m+ commands
instead even fail because the modules haven't been loaded yet.
Enable cabal repl exe:pts and add a wrapper

The syntax is less than intuitive, so let's save it. I plan to write
some docs.

Without these extra Build-depends, cabal repl exe:pts fails looking for
those modules.